One method for forming seamless titanium or titanium alloy pipe, albeit in short sections, is disclosed in International patent application PCT/AU2009/000276 in the name of Commonwealth Scientific and Industrial Research Organisation (“CSIRO”), which is incorporated by reference herein. The contents of the CSIRO International application are incorporated herein by this reference.
This method utilises cold spray technology which involves spraying particles at high velocities onto a substrate to cause the particles to bond together. Typically the particles are accelerated to supersonic velocities to cause bonding on impact. The term “cold” arises because the method is carried out at temperatures below the melting point of the particles and of the substrate. Accordingly, the original structure and properties of the particles are maintained throughout the process. Therefore, pipe formed of the particles will have similar properties. The general concept of cold-spraying and an example of a cold-spraying apparatus are disclosed in U.S. Pat. No. 5,302,414. which is incorporated by reference.
The method disclosed in the CSIRO International application involves spraying fine particles of titanium or titanium alloy onto a cylindrical mandrel that is rotated about a longitudinal axis of the mandrel.
The particles are sprayed at supersonic velocities and bond together upon impact with the mandrel to build up a layer of titanium or titanium alloy depending on the composition of the particles. The extent of the build-up determines the wall thickness of the pipe.
The spray nozzle is arranged to direct particles along a radial line of the mandrel so the particles impact a curved surface of the mandrel generally perpendicular to the surface of the mandrel. The nozzle is moved lengthwise relative to the rotating mandrel so that particles are sprayed over length of the mandrel and so that a pipe is formed with substantially the same length as the mandrel. The mandrel is finally separated from the formed pipe.
An advantage of the method disclosed in the CSIRO International application is that the layer is continuous over the length of the cylindrical mandrel so that the layer forms a seamless pipe. The band strength of the pipe is, therefore, well suited to high pressure applications.
While the pipe is seamless, the overall length of the pipe is limited by the length of the mandrel. As there are practical limitations on the lengths of mandrels, pipes are also limited in length. Accordingly, this CSIRO method does not produce titanium or titanium alloy pipe in lengths that are practical as a replacement for long sections of steel pipe.
Short sections of titanium or titanium alloy pipe may be used to form an overall longer pipe. However, there is considerable cost involved in assembling a longer pipe because the short sections are difficult to weld together. While the CSIRO method can produce titanium or titanium alloy pipe more cost effectively than other methods, this benefit is off-set to an extent by the cost and difficulty associated with assembling longer lengths of pipe from a series of short section of seamless titanium pipe.
Accordingly, there is a desire to produce longer sections of titanium or titanium alloy pipe to reduce the practical disadvantages of assembling short sections.
Summary of Disclosure
In a first aspect, a method of forming sections of seamless titanium or titanium alloy pipe is provided. The method comprises the steps of:                (a) providing a substrate for forming pipe and a sleeve of a section of pipe on the substrate, the pipe section having an end from which the substrate projects;        (b) spraying particles of titanium or titanium alloy generally parallel to a longitudinal axis of the substrate to impact an end face of the end and to cause particles to bond to and to accumulate on the pipe end face to form pipe; and        (c) moving formed pipe longitudinally relative to the substrate to remove formed pipe from the pipe-forming substrate and continuing to spray titanium or titanium alloy particles onto the end face to cause further pipe to form continuously and integrally with the formed pipe, thereby enabling formation of a seamless titanium or titanium alloy pipe of a desired length.        
The term “pipe-forming substrate” is a reference to a surface portion of a substrate. An underlying portion of the substrate may be formed of a different material, include heating or cooling structures or may be hollow.
The method enables formation of titanium or titanium alloy pipe to a desired length because the pipe is continuously formed and removed from the substrate. For practical purposes, the pipe may be formed in lengths suitable for transport, such as up to 16 meters or longer, or may be continuously formed and cut to predetermined lengths after the desired length has moved from the substrate during the forming process.
This method enables pipes to be formed with an internal diameter in the range of 1 mm to 1000 mm (typically). In addition, this method enables pipes to be formed with a wall thickness in the range of 0.1 mm to 50 mm (typically).
The method preferably involves evenly distributing sprayed particles over the face to cause even growth of the formed pipe by rotating the substrate and pipe relative to the particle spray.
Step (b) may involve spraying particles onto the end face via a plurality of spray nozzles.
The method may further comprise compressing formed pipe to reduce porosity of the formed pipe.
Research work carried out by the applicant has revealed that spray-formed pipe has pores associated with gaps between particles that impact and form the pipe. The porosity negatively impacts the overall strength of the pipe because the pores act as stress centre and assists to propagate cracks. However, the research work revealed that the porosity is reduced by compressing the formed pipe.
Suitable forces to compress the formed pipe vary depending on the depth of the deposited titanium layer between repetitive application of the force. This in turn depends on the speed of deposition, the rotational speed, the wall section thickness and the diameter of the pipe. Nevertheless, suitable compressive forces—pressure—are in the range of 10 to 1000 MPa.
The compressive force may be applied to an outwardly facing, circumferential curved surface of the pipe.
However, the compressive force is preferably applied to the location of particle accumulation. It is also preferable for the compressive force to be applied in the same direction that particles are sprayed.
Accordingly, the compressive force preferably is applied to the end face of the pipe.
Preferably, the compressive force is applied by a fixed roller to accumulated particles on the end face, whereby growth of the pipe causes longitudinal movement of formed pipe relative to the substrate.
The method further comprises controlling the compressive force.
The compressive force may be controlled by controlling friction between the substrate and the formed pipe, and optionally the pipe section or applying a load on the pipe opposite to the compressive load.
Controlling the friction may comprise selecting a substrate to provide sufficient friction to longitudinal movement of the formed pipe so that the compressive force applied by the roller causes compression of accumulated particles.
The applicant recognises that friction between the formed pipe and the substrate is affected by the extent to which the formed pipe bonds to the substrate. Without wishing to be bound by any particular theory, experimental work carried out by the applicant suggests, and it is the belief of the applicant, that bonding is affected by the following factors:                (a) differential thermal expansion of the formed pipe and the substrate;        (b) surface roughness of the substrate;        (c) chemical bonding between the formed pipe and the substrate; and        (d) titanium or titanium alloy “particle relaxation”.        
It is not clear to what extent each of these aspects contribute to overall bonding between the formed pipe and the substrate.
In view of this belief, the applicant anticipates that movement of the formed pipe relative to the substrate may be achieved by controlling the extent of bonding between the formed pipe and the pipe-forming substrate. It should be understood, however, that alternative options that enable movement of the formed pipe relative to the pipe-forming substrate are encompassed by the subject invention.
With the above factors in mind, the method may involve controlling the extent of bonding between formed titanium or titanium alloy pipe and the substrate to enable formed pipe to be moved relative to the substrate.
In regard to factor (a), it is believed that thermal bonding occurs when the titanium or titanium alloy particles are sprayed onto the substrate. In particular, it is believed that thermal bonding occurs if the substrate expands more than the formed pipe in the course of being exposed to the spray carrier gas which is at an elevated temperature.
One option for counteracting the thermal bonding effect may involve controlling the extent of bonding by heating the formed titanium or titanium alloy pipe to cause differential thermal expansion of the formed pipe relative to the substrate, thereby releasing the formed pipe from the pipe forming substrate and enabling the formed pipe to be moved relative to the substrate. The thermal differential may be caused preferentially by heating the formed titanium or titanium alloy pipe. Alternatively, the thermal differential may be caused by cooling the substrate causing a thermal differential between the formed pipe and the pipe forming substrate relative to the pipe.
Another option for counteracting the thermal bonding effect may involve controlling the extent of bonding by selecting a substrate having a co-efficient of thermal expansion that is less than the co-efficient of thermal expansion of the titanium or titanium alloy.
The substrate may be ceramic, glass, metal or composite.
In regard to factor (b), the applicant believes that surface morphology of the substrate affects the extent of bonding with the formed pipe. In particular, the applicant believes that the titanium or titanium alloy particles fill surface relief on the substrate with the effect that, at least in the longitudinal direction along the substrate, the formed pipe and the substrate mechanically interlock.
Accordingly, the applicant further believes that an option for reducing the impact of surface morphology and, hence overall bonding, may involve controlling the extent of bonding by selecting a substrate having a surface roughness to reduce mechanical bonding between the formed pipe and the substrate.
The average surface roughness may be Ra<1.0 μm. Preferably, the surface roughness may be Ra<0.5 μm.
In regard to factor (c), the applicant believes that bonding may be affected by the chemical affinity of the formed pipe to the pipe-forming substrate.
The substrate may be formed of a material that has little or no chemical potential for bonding with titanium or titanium alloy.
In regard to factor (d), the applicant believes that bonding is affected by mechanical reactions of titanium or titanium alloy alloys particles impacting on the pipe-forming substrate or on a section of forming pipe. Again, without wishing to be bound by any particular theory, the applicant believes that the titanium or titanium alloy particles elastically deform on impact by flattening to an extent. For example, generally spherical particles deform to produce a disc or elongated shape. It is though that, while in that deformed shape, the particles are impacted with and bind with other particles that are also elastically deformed. After impact and binding, the resiliency of the elastic particles provides a tendency to revert to their original shape. The particles, however, bind together while in an expanded shape so the resilience manifests as a contraction of the formed pipe about the substrate.
For convenience, this effect will be referred to by the term “particle relaxation”.
Spraying particles of titanium or titanium alloy in steps (b) and (c) may be in accordance with a cold-spray process disclosed in the CSIRO International application in order to form titanium or titanium alloy pipe.
In a second aspect, there is provided a titanium or titanium alloy pipe formed in accordance with the continuous forming method defined above.
The titanium or titanium alloy pipe may have a composition comprising:
titanium: 99.8 wt %; and
the balance comprising incidental impurities.
The titanium alloy pipe may alternatively have a composition comprising:
titanium: 90 to 94 wt %; and
aluminium and vanadium: 6 to 10 wt %; and
the balance comprising incidental impurities.
This does not exclude other alloys of titanium where titanium is greater than any other single element either by atom or by weight.
The pipe may be formed by spraying particles selected to have different compositions. For example, pipe may be formed in accordance with the first aspect from particles having one or more different alloy compositions. Alternatively, the pipe may be formed of particles of titanium and particles of one or more different alloy compositions.
In these circumstances, the pipe may be formed with a generally homogenous composition or the pipe may be formed with a composition that is graded or otherwise varies along the length of the pipe.
In a third aspect, there is provided an apparatus for spray-forming pipe in accordance with the first aspect. The apparatus comprises:                (a) a substrate for forming pipe, the substrate being fitted with a sleeve of a section of pipe having an end face;        (b) means for rotating the substrate and pipe section about a longitudinal axis of the substrate; and        (c) means for cold-spraying particles of titanium or titanium alloy generally parallel to the longitudinal axis to impact the end face and to form seamless pipe.        
The apparatus may further comprise compression means for applying a compressive force to the end face to compress particles that accumulate on the end face and to cause longitudinal movement of the formed seamless pipe relative to the substrate.
The substrate is preferably selected to provide sufficient friction to cause compression of accumulated particles by the compression means and to cause the longitudinal movement of the formed pipe relative to the substrate.
The apparatus may further comprise means for applying a compressive force to the formed pipe in a radial direction with respect to the direction of rotation of the substrate.